1. Field of the Invention
This invention relates to the manufacture of vinyl acetate using a catalyst comprising palladium and gold. More particularly, the invention is directed to a method for pretreating a metallic palladium-gold catalyst by sequential oxidation and reduction steps.
2. Discussion of the Art
The process of employing palladium-gold catalysts to obtain vinyl acetate from ethylene, acetic acid, and oxygen has been known for decades. Such catalysts are discussed by Sennewald et al in U.S. Pat. No. 3,488,295 and U.S. Pat. No. 3,743,607, and by Nakamura et al. in U.S. Pat. No. 4,087,622.
The catalytic elements in palladium-gold systems are in substantially metallic form, although some oxides may be present at the surface. Gold is typically present in minor amounts relative to the amount by weight of palladium. These expensive metals are deposited in relatively minor amounts on the surface of a support material or a carrier such as silica, alumina, or other materials. Examples of support materials are given by Bartsch in U.S. Pat. No. 4,158,737 and by Wunder et al. in U.S. Pat. No. 4,902,823.
There are various methods for depositing the palladium and gold on the carrier surface. One is described in U.S. Pat. No. 3,917,676 to Kisaki et al where in Example 1 a palladium salt is first dissolved, an alumina carrier is impregnated with that solution, the coated carrier is dried in air at 250.degree. C., and palladium metal is finally obtained by reducing with hydrogen at 200.degree. C. It is also possible, as mentioned in U.S. Pat. No. 4,087,622 to Nakamura and in U.S. Pat. No. 3,917,676 to Kisaki, to heat the metal salt to cause the corresponding oxide to be formed in situ on the support, and then to reduce the oxide to form the metal.
These and similar methods of catalyst preparation which involve sequential oxidation and reduction of metal salts to manufacture a catalyst in metallic form are to be distinguished from the present invention for pretreatment in which a finished catalyst, already in metallic form, is then subjected to the oxidation and reduction treatment of the invention.
Procedures used to regenerate spent catalysts must also be distinguished. It is well known that vinyl acetate catalysts will eventually decline in activity to a point where it becomes economic to regenerate the catalyst and/or replace it with fresh catalyst. Lum and Mador in U.S. Pat. No. 3,480,558 teach the regeneration of a spent palladium catalyst by sequential treatment with oxygen and hydrogen to remove organic residues from the catalyst. There is no suggestion that such treatment of a virgin catalyst would have any beneficial effect.
It is an object of this invention to achieve a catalyst with improved initial selectivity to vinyl acetate while maintaining a high conversion. It is also an object to reduce the frequency of regeneration or replacement of the catalyst.